I have had issues for several months with the thumbnails on PDF icons only. They are sometimes transparent, solid black, or have the PDF icon only. I have uninstalled and reinstalled many times. I have "enable PDF thumbnails in Windows Explorer" selected. Adobe Acrobat DC Reader is my default program with version 2021.001.20135. I have tried many things over a while, as this really was bugging me. Tonight I installed "Fixes for 64-bit Adobe Reader preview handler and thumbnails" from:
https://www.pretentiousname.com/adobe_pdf_x64_fix/index.html#thank

I figured I had nothing to lose and surprisingly it fixed the issue, especially as it hasn't been updated since 2011.

Please build into Adobe Acrobat DC Reader a means to reset Adobe's PDF preview handler and thumbnails on 64-bit versions of Windows. Uninstalling and reinstalling doesn't fix it. Believe me...I have tried.